ABSTRACT: Extensional Attenuation Detachment Faulting: A Possible Structural Model for Pre-Tertiary Rocks Beneath Railroad Valley
WALKER, CHARLES T., ROBERT D. FRANCIS, and WILLIAM W. LUMSDEN, California State University at Long Beach, Long Beach, CA
Many investigators have suggested that low-angle, younger-over-older faults in eastern Nevada are the response of a brittle suprastructure
to ductile extension in a metamorphic infrastructure. A detachment marks the boundary between the two structural styles.
Although low-angle normal faults occur in the inferred brittle suprastructure of the White Pine and Schell Creek ranges of east-central Nevada, true detachment faults also occur in ductile shales well above the inferred brittle-ductile transition. Tectonic topography associated with these detachments leads to complex field relations. At various locations, pre-Chainman rocks ranging in age from early Mississippian to Cambrian are in tectonic contact with Chainman Shale, which commonly is topographically lower, a field relation normally associated with thrusting. Nevertheless, wherever this contract is exposed, Chainman Shale overlies the older rocks with tectonic discontinuity, proving that the tectonic contact is a detachment. Extension of some brittle units between shale detachments y low-angle normal faulting has produced isolated brittle lenses. Between the Duck Creek and Schell Creek ranges, extension has attenuated pre-Carboniferous rocks and created a structural basin that can be traced northward into a typical graben. This, plus the fact that Basin and Range faulting cuts attenuated structure in the White Pine Range, suggests that attenuation may play a role in initiating Basin and Range structures. Therefore, structures within the White Pine and Schell Creek ranges may serve as guides to structures in the Paleozoic and Tertiary rocks that reservoir oil in Railroad Valley, and to the distribution of ductile shales with source potential.